The Sharpie should stay on quite well if you let the figure sit for about a day or two (maybe a bit of smearing but just be careful). Do not touch it right after you have applied the Sharpie because it will smear and create a huge color blob all over your minifig. If you make a mistake when using the sharpie, quickly rub it off and use a bit of water to get the mark completely off. Thanks for the good comment!